[ed note: The diagram mentioned below is not yet available online.]
The diagram at the centrefold of this book shows the location of the Macauley Point and Clover Point outfalls and a cross section at the points of discharge. It has been distorted to simplify explanations but that does not invalidate the principle that it illustrates. In reality the rising column of sewage is much narrower in relation to its height than the diagram indicates; also it will be skewed to one side or the other depending on the currents.
It will be seen that the sewage discharges a little under a mile from shore in the one case and a little over a mile in the other at a depth of about 200 feet, which is greater than Victoria's tallest buildings. The force that drives the column upwards is the temperature and salinity difference between the sewage and the surrounding sea. The extreme cold in the Straits year round ensures that the column pours upwards rapidly. As it does so it entrains sea water so the column becomes closer and closer to the composition of the sea as it rises. Before this driving force has expended itself the column is only one half of one percent sewage, at the most, and is commonly one tenth of one percent. These figures may be compared with the effluent discharging from a secondary treatment plant which will usually be at ten percent of the incoming sewage strength. Indeed, a sample taken a little way up the vertical column will already be superior to such an effluent. Furthermore a discharge of secondary treated effluent through a short outfall would be worse than the raw sewage discharging down the long outfalls.
The column may be thought of as a vertical tank, bounded by the laws of physics as effectively as a land-based treatment tank is bounded by concrete but bigger, more efficient, requiring no power and not subject to any form of failure.
The hugely diluted mass then disperses horizontally, rapidly reaching a state that is indistinguishable from sea water, even under the most acute tests. Such a test is the 'coliform' test, already mentioned, which can detect coliforms at less than one millionth of the strength of the incoming sewage. No test has yet revealed coliforms along our shorelines from the long outfalls, although there are plenty of coliforms from other discharges.
This state of affairs may be compared with the discharge of secondary treated effluent to a stream or river, which is a perfectly acceptable practice. The effluent, effectively diluted ten fold as explained, will there discharge to the watercourse which will dilute it further, depending on the ratio of the river water to the amount of effluent.
In the one case a liquid mass one two hundredth to one thousandths of the strength of sewage discharges one mile from shore into salt water. In the other a liquid mass one tenth of the strength of sewage discharges to fresh water, which pathogens much prefer, some feet from the adjoining banks. Commonly there will be property near the river some little way downstream. There are numerous such discharges doing no harm to the environment nor to man. How can our sea discharges be doing harm?